Why is it illegal for Google's autonomous car to drive on the road by itself?
The question is based on a false fact: in Michigan, it is currently legal (under certain conditions described here ) for an autonomous car to operate without a driver. 

The reason that the federal government has not enacted any direct legislation (although they have enacted guidelines) on autonomous cars is because it is still a developing technology (as great as those Cruise videos look, I wouldn't trust most companies working on the technology to have driverless cars on the road without operators) and that it arguably falls under the state governments jurisdictions. Some companies have appealed to the House to pass national legislation to allow autonomous testing (GM, Toyota, Lyft), but nothing has come of it yet.